1. Field of the Invention
The present invention relates to a broadband wireless access communication system, and more particularly to a system and method for performing a handover operation upon receipt of an SS (Subscriber Station) request in a BWA (Broadband Wireless Access) communication system using an OFDM (Orthogonal Frequency Division Multiplexing) scheme.
2. Description of the Related Art
Intensive research is being conducted into the 4G (4th Generation) communication system, one of the next generation communication systems, to provide a plurality of users with specific services having a variety of QoSs (Quality of Services) at a transfer rate of about 100 Mbps. Presently, the 3G (3rd Generation) communication system provides a transfer rate of about 384 kbps in an outdoor channel environment having a relatively poor channel environment, and provides a maximum transfer rate of about 2 Mbps in an indoor channel environment having a relatively good channel environment. A wireless Local Area Network (LAN) system and a wireless Metropolitan Area Network (MAN) system have been designed to provide a transfer rate of 20˜50 Mbps. The 4G communication system provides the wireless LAN and MAN systems a relatively high transfer rate with mobility and QoS, and many secondary developers are conducting intensive research into high-speed services to be provided from the 4G communication system.
However, the wireless MAN system is suitable for a high-speed communication service in that it has a wide coverage area and supports a high-speed transfer rate, but it does not consider the mobility of a subscriber station (SS) at all, so that there is no consideration of the need for a handover operation (i.e., a cell selection operation) caused by the movement of the SS. The communication system currently considered in IEEE (Institute of Electrical and Electronics Engineers) 802.16a specification acts as a specific communication system for performing a ranging operation between the SS and a base station (BS). The communication system considered in the IEEE 802.16a specification will hereinafter be described with reference to FIG. 1.
FIG. 1 is a block diagram illustrating a BWA communication system using an OFDM/OFDMA (Orthogonal Frequency Division Multiplexing/Orthogonal Frequency Division Multiple Access) scheme. In more detail, FIG. 1 depicts the IEEE 802.16a communication system.
The wireless MAN system acting as a BWA communication system has a much wider coverage area and a much higher transfer rate than the wireless LAN system. In case of adapting the OFDM scheme and the OFDMA scheme to a physical channel of the wireless MAN system to provide the wireless MAN system with a broadband transmission network, this application system is referred to as an IEEE 802.16a communication system. The IEEE 802.16a communication system applies the OFDM/OFDMA scheme to the wireless MAN system, such that it transmits a physical channel signal using a plurality of sub-carriers, resulting in high-speed data transmission. The IEEE 802.16e communication system has been designed to consider the mobility of an SS in the IEEE 802.16a communication system. There is no detailed specification for the IEEE 802.16e communication system.
Referring to FIG. 1, the IEEE 802.16a communication system has a single cell structure, and is composed of a BS 100 and a plurality of SSs 110, 120, and 130 managed by the BS 100. Signal transmission/reception among the BS 100 and the SSs 110, 120, and 130 can be established using the OFDM/OFDMA scheme. A downlink frame structure for use in the IEEE 802.16a communication system will hereinafter be described with reference to FIG. 2.
FIG. 2 is a conceptual diagram illustrating the downlink frame structure for use in the BWA communication system using the OFDM/OFDMA scheme. In more detail, FIG. 2 depicts a downlink frame structure for use in the IEEE 802.16a communication system.
Referring to FIG. 2, the downlink frame includes a preamble field 200, a broadcast control field 210, and a plurality of TDM (Time Division Multiplexing) fields 220 and 230. A synchronous signal (i.e., a preamble sequence) for acquiring synchronization between the BS and the SSs is transmitted via the preamble field 200. The broadcast control field 210 is composed of a DL(DownLink)_MAP field 211 and a UL(UpLink)_MAP field 213. The DL_MAP field 211 is adapted to transmit the DL_MAP message, and a plurality of IEs (Information Elements) contained in the DL_MAP message are shown in the following Table 1:
TABLE 1SyntaxSizeNotesDL_MAP_Message_Format( ){Management Message Type=2 8 bitsPHY Synchronization FieldVariableSee appropriate PHYspecificationDCD Count 8 bitsBase Station ID48 bitsNumber of DL_MAP Element n16 bitsBegin PHY Specific section {See applicable PHYsection for (I=1; i<=n; i++)For each DL_MAPelement 1 to nDL_MAP InformationVariableSee correspondingElement( )PHY specification if! (byte boundary) {  Padding Nibble 4 bitsPadding to reach byteboundary     }    }   }  }
With reference to the above Table 1, the DL_MAP message includes a Management Message Type field indicative of a plurality of IEs (i.e., transmission message type information); a PHY (PHYsical) Synchronization field established in response to a modulation or demodulation scheme applied to a physical channel in order to perform synchronization acquisition; a DCD count field indicative of count information in response to a DCD (Downlink Channel Descriptor) message configuration variation containing a downlink burst profile; a Base Station ID field indicative of a Base Station Identifier; and a Number of DL_MAP Element n field indicative of the number of elements found after the Base Station ID. Particularly, the DL_MAP message (not shown in Table 1) includes information associated with ranging codes allocated to individual ranging processes to be described later.
The UL_MAP field 213 is adapted to transmit the UL_MAP message, and a plurality of IEs contained in the UL_MAP message are shown in the following Table 2:
TABLE 2SyntaxSizeUL_MAP_Message_Format( ){ Management Message Type=3 8 bits Uplink Channel ID 8 bits UCD Count 8 bits Number of UL_MAP Element n16 bits Allocation Start Time32 bits Begin PHY Specific section {  for (i=1; i<n; i+n)  UL_MAP Information_Element( )Variable    Connection ID    UIUC    Offset   }  } }}
With reference to Table 2, the UL_MAP message includes a Management Message Type field indicative of a plurality of IEs (i.e., transmission message type information); an Uplink Channel ID field indicative of a used Uplink Channel ID; a UCD (Uplink Channel Descriptor) count field indicative of count information in response to a UCD message configuration variation containing an uplink burst profile; and a Number of UL_MAP Element n field indicative of the number of elements found after the UCD count field. In this case, the uplink channel ID can only be allocated to a Media Access Control (MAC) sub-layer.
The UIUC (Uplink Interval Usage Code) area records information indicative of the usage of offsets recorded in the offset area. For example, provided that 2 is recorded in the UIUC area, a starting offset for use in the initial ranging process is recorded in the offset area. Provided that 3 is recorded in the UIUC area, a starting offset for use in either the bandwidth request ranging or the maintenance ranging process is recorded in the offset area. The offset area records a starting offset value for use in either the initial ranging process or the maintenance ranging process according to the information recorded in the UIUC area. Physical channel characteristic information to be transferred from the UIUC area is recorded in the UCD.
Provided that the SS results in a ranging failure, a predetermined backoff value is set up to increase the probability of success in the next trial, and the ranging process is re-performed after the lapse of a predetermined time corresponding to the backoff time. In this case, information needed for determining the backoff value is contained in the UCD message. The aforementioned UCD message configuration is shown in the following Table 3:
TABLE 3SyntaxSizeNotesUCD-Message Format( ){Management Message Type=08 bitsUnlink channel ID8 bitsConfiguration Change Count8 bitsMini-slot size8 bitsRanging Backoff Start8 bitsRanging Backoff End8 bitsRequest Backoff Start8 bitsRequest Backoff End8 bitsTLV Encoded Information forVariablethe overall channelBegin PHY Specific Section { for (i=1; i<n; i+n)  Uplink_Burst_DescriptorVariable  } }}
With reference to the Table 3, the UCD message includes a Management Message Type field indicative of a plurality of IEs (i.e., transmission message type information); an Uplink Channel ID field indicative of a used Uplink Channel Identifier; a Configuration Change Count field counted by the BS; a mini-slot size field indicative of the number of mini-slots of the uplink physical channel; a Ranging Backoff Start field indicative of a backoff start point for an initial ranging process, i.e., an initial backoff window size for the initial ranging process; a Ranging Backoff End field indicative of a backoff end point for the initial ranging process, i.e., a final backoff window size; a Request Backoff Start field indicative of a backoff start point for establishing contention data and requests, i.e., an initial backoff window size; and a Request Backoff End field indicative of a backoff end point for establishing contention data and requests, i.e., a final backoff window size. In this case, the backoff value indicates a kind of standby time which is a duration time between the start of SS's access failure and the start of SS's re-access time. If the SS fails to execute an initial ranging process, the BS must transmit the backoff values indicative of standby time information for which the SS must wait for the next ranging process to the SS. For example, provided that a specific number of 10 is determined by the “Ranging Backoff Start” and “Ranging Backoff End” fields shown in Table 3, the SS must pass over 210 access executable chances (i.e., 1024 access executable chances) and then execute the next ranging process according to a Truncated Binary Exponential Backoff Algorithm.
The TDM fields 220 and 230 indicate fields corresponding to timeslots allocated using a TDM/TDMA (Time Division Multiplexing/Time Division Multiple Access) scheme. The BS transmits broadcast information to be broadcast to SSs managed by the BS over the DL_MAP field 211 using a predetermined center carrier. The SSs monitor all the frequency bands having been previously allocated to individual SSs upon receipt of a power-on signal, such that they detect a pilot channel signal having the highest signal intensity, i.e., the highest pilot CINR (Carrier to Interference and Noise Ratio). It is determined that the SS belongs to a specific BS which has transmitted the pilot channel signal with the highest pilot CINR. The SSs check the DL_MAP field 211 and the UL_MAP field 213 of the downlink frame having been transmitted from the BS, such that they recognize their own uplink and downlink control information and specific information indicative of a real data transmission/reception position.
The downlink frame structure for use in the IEEE 802.16a communication system has been disclosed with reference to FIG. 2. An uplink frame structure for use in the IEEE 802.16a communication system will hereinafter be described with reference to FIG. 3.
FIG. 3 is a conceptual diagram illustrating an uplink frame structure for use in a BWA communication system using an OFDM/OFDMA scheme. In more detail, FIG. 3 depicts an uplink frame structure for use in the IEEE 802.16a communication system.
Prior to describing the uplink frame structure shown in FIG. 3, three ranging processes for use in the IEEE 802.16a communication system, i.e., an initial ranging process, a maintenance ranging process (also called a period ranging process), and a bandwidth request ranging process will hereinafter be described in detail.
First, the initial ranging process will be described in detail.
The initial ranging process for establishing synchronization acquisition between the BS and the SS establishes a correct time offset between the SS and the BS, and is adapted to control a transmission power (also called a transmit power). In more detail, the SS is powered on, and receives the DL_MAP message, the UL_MAP message, and the UCD message to establish synchronization with the BS in such a way that it performs the initial ranging process to control the transmission power between the BS and the time offset. In this case, the IEEE 802.16a communication system uses the OFDM/OFDMA scheme, such that the ranging procedure requires a plurality of ranging sub-channels and a plurality of ranging codes. The BS allocates available ranging codes to the SS according to objectives of the ranging processes (i.e., the ranging process type information). This operation will hereinafter be described in detail.
The ranging codes are created by segmenting a PN (Pseudorandom Noise) sequence having a length of 215-1 bits into predetermined units. Typically, one ranging channel is composed of two ranging sub-channels each having a length of 53 bits, PN code segmentation is executed over the ranging channel having the length of 106 bits, resulting in the creation of a ranging code. A maximum of 48 ranging codes RC#1˜RC#48 can be assigned to the SS. More than two ranging codes for every SS are applied as a default value to the three ranging processes having different objectives, i.e., an initial ranging process, a period ranging process, and a bandwidth request ranging process. In this way, a ranging code is differently assigned to the SS according to each objective of the three ranging processes. For example, N ranging codes are assigned to the SS for the initial ranging process as denoted by a prescribed term of “N RC (Ranging Codes) for Initial Ranging”, M ranging codes are assigned to the SS for the periodic ranging process as denoted by a prescribed term of “M RCs for maintenance ranging”, and L ranging codes are assigned to the SS for the bandwidth request ranging process as denoted by a prescribed term of “L RCs for BW-request ranging”. The assigned ranging codes are transmitted to the SSs using the DL_MAP message, and the SSs perform necessary ranging procedures using the ranging codes contained in the DL_MAP message.
Second, the period ranging process will be described in detail.
The period ranging process is periodically executed such that an SS which has controlled a time offset between the SS and the BS, and a transmission power in the initial ranging process can control a channel state associated with the BS. The SS performs the period ranging process using the ranging codes assigned for the period ranging process.
Third, the bandwidth request ranging process will be described.
The bandwidth request ranging process is adapted to enable the SS, which has controlled a time offset between the SS and the BS, and a transmission power in the initial ranging process, to request a bandwidth allocation from the BS in such a way that the SS can communicate with the BS.
Referring to FIG. 3, the uplink frame includes an initial maintenance opportunity field 300 using the initial and period ranging processes, a request contention opportunity field 310 using the bandwidth request ranging process, and an SS scheduled data field 320 composed of uplink data of a plurality of SSs. The initial maintenance opportunity field 300 includes a plurality of access burst fields each having real initial and period ranging processes, and a collision field in which there is a collision between the access burst fields. The request contention opportunity field 310 includes a plurality of bandwidth request fields each having a real bandwidth request ranging process, and a collision field in which there is a collision between the bandwidth request ranging fields. The SS scheduled data fields 320 are each composed of a plurality of SS scheduled data fields (i.e., SS 1 scheduled data field˜SS N scheduled data field). The SS transition gap is positioned between the SS scheduled data fields (i.e., SS 1 scheduled data field˜SS N scheduled data field).
FIG. 3 has disclosed the uplink frame structure for the IEEE 802.16a communication system. A ranging procedure for the IEEE 802.16a communication system using an OFDM scheme will hereinafter be described with reference to FIG. 4.
FIG. 4 is a flow chart illustrating the ranging procedure between the SS and the BS in a BWA communication system using the OFDM scheme.
Referring to FIG. 4, the SS 400 monitors all of its own predetermined frequencies upon receipt of a power-on signal, such that it detects a pilot channel signal having the highest signal intensity, i.e., the highest pilot CINR (Carrier to Interference and Noise Ratio). It is determined that SS 400 belongs to a specific BS 420 which has transmitted the pilot channel signal with the highest pilot CINR. The SS 400 receives a downlink frame preamble from the BS 420, such that it acquires system synchronization with the BS 420.
Upon establishing synchronization between the SS 400 and the BS 420, the BS 420 transmits the DL_MAP message and the UL_MAP message to the SS 400 at steps 411 and 413, respectively. As previously shown in Table 1, the DL_MAP message includes a variety of information, for example, requisite information for establishing synchronization between the SS 400 and the BS 420 in a downlink direction and configuration information of a physical channel capable of receiving a variety of messages transmitted to a plurality of SSs 400 over a downlink channel. As previously shown in Table 2, the UL_MAP message informs the SS 400 of SS scheduling interval information and physical channel configuration information, etc.
The DL_MAP message is periodically broadcast from the BS to all the SSs. In the case where the SS 400 can continuously receive the periodically-broadcast DL_MAP message, it is assumed that the SS is synchronized with the BS. The SSs receiving the DL_MAP message can receive all the messages transmitted over a downlink channel.
As stated above in Table 3, if the SS results in an access failure, the BS transmits the UCD message containing available backoff value indication information to the SS.
In case of performing the above ranging process, the SS 400 transmits an RNG_REQ (Ranging Request) message to the BS 420 at step 415. The BS 420 receiving the RNG_REQ message transmits an RNG_RSP (Ranging Response) message containing information for controlling a variety of factors (e.g., frequency, time, and transmission power) to the SS 400 at step 417.
The RNG_REQ message configuration is shown in the following Table 4:
TABLE 4SyntaxSizeNotesRNG-REQ_Message_Format( ){Management Message Type=48 bitsDownlink Channel ID8 bitsPending Until Complete8 bitsTLV Encoded InformationVariableTLV specific
With reference to Table 4, the Downlink Channel ID field indicates a downlink channel ID contained in the RNG-REQ message received in the SS via the UCD. The Pending Until Complete field indicates priority information of a transmission ranging response. In more detail, if the Pending Until Complete field is set to “0”, a previous ranging response has priority. Otherwise, if the Pending Until Complete field is not set to “0”, a current transmission ranging response has priority.
A detailed configuration of the RNG_RSP message to the RNG_REQ message shown in Table 4 is shown in Table 5:
TABLE 5 SyntaxSizeNotesRNG_RSP_Message_Format( ){Management Message Type=58 bitsUplink Channel ID8 bitsTLV Encoded InformationVariableTLV specific
With reference to Table 5, the Uplink Channel ID field indicates an uplink channel ID contained in the RNG_REQ message.
FIG. 4 has disclosed the ranging process when the IEEE 802.16a communication system uses the OFDM scheme. A ranging procedure for the IEEE 802.16a communication system using the OFDMA scheme will hereinafter be described with reference to FIG. 5. In this case, the IEEE 802.16a communication system contains a dedicated ranging interval in order to enable the IEEE 802.16a communication system to more effectively perform the ranging process using the OFDMA scheme, so that it may transmit the Ranging-Code instead of the RNG_REQ message according to a ranging code transmission scheme in the dedicated ranging interval.
Referring to FIG. 5, the BS 520 transmits the DL_MAP message and the UL_MAP message to the SS 500 at steps 511 and 513, respectively. Detailed operations of steps 511 and 513 are equal to those of steps 411 and 413. The communication system using the OFDMA scheme of FIG. 5 transmits a ranging code instead of the RNG-REQ message having been described in FIG. 4 at step 515. The BS 520 receiving the ranging code transmits the RNG_RSP message to the SS 500 at step 517.
New information must be added to the RNG_RSP message such that information corresponding to the ranging code transmitted to the BS can be recorded in the RNG_RSP message. The aforementioned new information to be added to the RNG_RSP message is composed of a ranging code (i.e., a received ranging CDMA code), a ranging symbol (i.e., an OFDM symbol in the received ranging CDMA code), a ranging sub-channel (i.e., a ranging sub-channel in the received ranging CDMA code), and a ranging frame number (i.e., a frame number in the received ranging CDMA code).
As described above, the IEEE 802.16a communication system operates on the basis of a fixed state of a current SS (i.e., there is no consideration given to the mobility of the SS) and a single cell structure. However, the IEEE 802.16e communication system has been defined as a system for considering the SS's mobility in the IEEE 802.16a communication system, such that the IEEE 802.16e communication system must consider the SS's mobility in a multi-cell environment. In order to provide the SS's mobility in the multi-cell environment, individual operating modes of the SS and the BS must be converted. However, the IEEE 802.16e communication system has not proposed a new method for the SS's mobility in the multi-cell environment. In conclusion, a handover system considering both an idle state and a communication service execution mode must be developed to provide mobility to an SS in a IEEE 802.16e communication system.